Inkjet printers frequently include an inkjet print head mounted on a carriage that is moved back and forth across print media, such as paper. As the print head is moved across the print media, a control system activates the print head to deposit or eject ink droplets onto the print media to form images and text.
The print head typically includes a fluid reservoir that is fluidically coupled to a substrate that includes ink passageways and is attached to the back of a nozzle layer containing one or more nozzles through which fluid is ejected. The substrate includes energy-generating elements that generate the force necessary for ejecting the fluid held in the reservoir. Two widely used energy-generating elements are thermal resistors and piezoelectric elements. The former rapidly heats a component in the fluid above its boiling point to cause ejection of a drop of the fluid. The latter utilizes a voltage pulse to generate a compressive force on the fluid resulting in ejection of an ink drop.
Ink is provided to the print head by a supply of ink that is either carried by the carriage or is mounted to the printing system and does not to move with the carriage. Where the ink supply is carried with the carriage, referred to as “on-board” or “on-axis” ink supply, the ink supply can be integral with the print head, such that the entire print head and ink supply is replaced when ink is exhausted.
Alternatively, printers have been developed having moving print heads that are connected to stationary ink supplies. This development is called “off-axis” printing and allows the ink supply to be replaced as it is consumed, without requiring the frequent replacement of the costly print head containing the fluid ejectors and nozzle system. Where the ink supply is separately replaceable, the ink supply is replaced when exhausted, and the print head need not be replaced until the end of print head life.
Naturally, it is desirable that the ink supply provide a reliable supply of ink to the inkjet print head. Clogs in ink conduits and passageways and/or changes in ink viscosity can impede this reliability. The ink itself is a mixture including pigments or dyes and other substances carried in a solvent base. The precise mixture of a given ink is carefully designed to provide certain desired qualities of appearance, durability, etc. after the ink is applied to a substrate and dries. The characteristics of the liquid ink are also important because they can affect the accuracy and efficiency of the ink delivery system. For example, the viscosity of a liquid ink is one parameter that can affect the pump metering accuracy of the delivery system, and thereby affect printing quality. Similarly, a high negative pressure on the inlet side of a peristaltic pump, e.g. caused by a clog, can also affect pump metering accuracy.
One challenge that must be dealt with in an ink delivery system is the potential for clogs in ink conduits and passageways, including the nozzles of the print head orifice plate. After an extended idle period, ink within an ink delivery system can gradually lose solvent, such that it either forms a solid obstruction, thereby preventing flow, or produces an increase in viscosity such that dynamic flow losses affect the pump metering accuracy. In a multi-color ink delivery system, such as a color printer that draws ink from multiple reservoirs of different colors, an obstruction or flow reduction associated with just one of the ink colors and one of the pens can significantly affect print quality, and/or result in substantial down time, lost productivity and expense while the problem is corrected.
Some proposed solutions for clearing clogs involve the activation of portions of existing pump recharge and purge cycles. However, because pump metering accuracy is likely affected while a flow obstruction is in place, portions of a pump cycle may be unsuccessful, potentially causing loss of backpressure in the print head or other problems.